1. Field of the Invention
This invention relates to photometric determinations of sulfur content of hydrocarbon compositions containing sulfur. More specifically, this invention is directed to chemiluminescence determinations and their utility in determining sulfur content.
2. Description of the Prior Art
R. L. Shearer, D. L. O'Neal, R. Rios, and M. D. Baker of Shell Development Company report in a paper entitled Analysis of Sulfur Compounds by Capillary Column Gas Chromatography with Sulfur Chemiluminescence Detection, published in Journal of Chromatographic Science, vol. 28, 24-28 (1990) that detection of sulfur content by means of sulfur chemiluminescence detection ("SCD") provides at least an order of magnitude improvement over flame photometric detection ("FPD") of sulfur. In fact, the article reports that an FID flame can be sampled directly into the reaction cell of an SCD system. The reaction cell stabilizes both the SO formed in the flame and reduces the interference of water and carbon dioxide by being run at low pressures. The amounts of SO produced will vary depending upon the air and hydrogen flow rates to the FID system burner. Upon the reaction of SO species with ozone a sulfur dioxide species forms that emits light in the wavelength region of 260 to 480 nanometers ("nm"). The optimal flow rates of hydrogen and air for FID and SCD are reported to be different.
R. L. Shearer reported in an article entitled, Development of Flameless Sulfur Chemiluminescence Detection: Application to Gas Chromatography, published in Analytical Chemistry, Vol. 64, No. 18, Sep. 15, 1992, pages 2192-6, the following problems: the SCD detector response was highly dependent on the condition and positioning of a ceramic probe that is used to sample the postflame gases from a flame ionization detector; and detector response could also be reduced when column bleed or other impurities accumulated on the probe (overcome by an inconvenient conditioning procedure). These problems were reportedly overcome by using an externally heated ceramic combustion assembly that is operated at low pressure and under fuel-rich conditions outside of the flammability limits of hydrogen and air. First a mixture of oxygen, make up and sample from a column are mixed and heated externally just prior to reaction in a combustion zone and then mixed with hydrogen in a space between a ceramic cylindrical core and outer walls. The resulting mixture is then sent through a transfer line to a chemiluminescence cell for reaction with ozone. The optimal thermal range reported was between 800.degree.-900.degree. C. The absolute signal increased with temperature, but so also did background noise. Air or oxygen could equally well be use, provided the stoichiometry of hydrogen and oxygen was the same.
Antek 6000 Process Nitrogen and Sulfur Analyzers, sold by Antek Industrial Instruments, Inc., Houston, Tex., is reported to be able to detect nitrogen and sulfur individually or simultaneously. Ultra-violet irradiation of the sample in a chamber and subsequent detection of the resulting fluorescence is used to determine the amount of sulfur present. Ozone is not used in any portion of the sulfur detection method.
Sievers Research, Inc., of Colorado, sells a Sievers Model 350 Sulfur Chemiluminescence System that utilizes SCD to determine the amount of sulfur present in a sample. The system uses a single stage hydrogen/air flame as the source for SO species.
R. L. Benner and D. H. Stedman report in an article entitled Universal Sulfur Detection by Chemiluminescence published in Ana. Chem. 1989, 61, 1268-1271, that both reduced and oxidized sulfur compounds could be measured simultaneously following sampling of a hydrogen flame into an ozone reactor at low pressures. Low pressure is required to ensure that no water vapor condenses, which condensation will interfere with ozone induced chemiluminescence. A drawing comparing a single flame system to a two stage system is shown in FIG. 2. Also disclosed are the criterion for hydrogen to oxygen ratios in the flame to maximize the signal to noise ratio. The fixed opening of a quartz sample tip that leads to the ozone reaction cell was varied as to distance from the hydrogen inlet to produce different residence times in the flame for a particular samples. R. L. Shearer reports in an article entitled, Development of Flameless Sulfur Chemiluminescence Detection: Application to Gas Chromatography, in Anal. Chem. 1992, 64, 2192-2196, that the SCD detector response is highly dependent on the condition and positioning of a ceramic probe that is used to sample the postflame gases from a flame ionization detector. This reference used a single stage pyrolysis process that preheated and combusted the gases prior to mixing with hydrogen at conditions that did not support a flame with the added hydrogen.
Although all reports concerning SCD indicate a substantial improvement over FPD, the following problems have been found in using SCD to measure sulfur content. When certain hydrocarbons are present in a sample being tested for sulfur content, there is an additional signal from some ozone induced reactions that do not involve sulfur, even though such hydrocarbons are first burned in a hydrogen/oxygen flame. Interestingly, hydrocarbons that contain oxygen are less likely to give rise to such signals. Some improvement in signal detection has been reported by N. Quickert et al. in an article entitled, Modification of a Chemiluminescence Ozone Monitor for the Measurement of Gaseous Unsaturated Hydrocarbons, published in The Science of the Total Environment, 3 (1975) 323-328 (Elsevier Scientific Publishing Company, Amsterdam, by selecting appropriate-light filters. However, even with such improvements interference from signals unrelated to sulfur content still occur. The sulfur chemistry in oxygen/hydrogen flames is believed to be very complicated (See article entitled Sulfur-Selective Detection with the FPD: Current Enigmas, Practical Usage, and Future Directions, by S. O. Farwell and C. J. Barinaga published in Journal of Chromatographic Science, Vol. 24, November 1986, pages 483-494). There are reported to be as many as thirteen equilibrium reactions, ten of which involve two interacting species, and three, involve three interacting species. Even ignoring the three reactions involving three species, the changes in sulfur distribution among the six possible sulfur containing species, i.e. HS, H.sub.2 S, SO, SO.sub.2, S, and S.sub.2, are expected to be determined by the kinetics of the overall bimolecular reaction scheme at each point in the flame; S.sub.2 may not be a dominant species in such flames; variations in H.sub.2 /O.sub.2 stoichiometry will also alter the sulfur distribution among the six species identified. J. C. Kramlich in his Ph.D. thesis from Washington State University, 1980, entitled, The Fate and Behavior of fuel-sulfur in combustion Systems, reported that the concentration of sulfur in SO and SO.sub.2 account for at least 85% of the total sulfur, whereas S.sub.2 was predicted to never exceed a few pads-per-million. As had been observed and reported, the location of sampling the hydrogen/oxygen rich flame for transport into the ozone reaction cell has a significant impact on the strength of chemiluminescence observed. This is consistent-with the variation in sulfur distribution among the five species at each point in such flames.
This invention is in part based upon the discovery, that despite the complexity of the reactions occurring, the temperature and hydrogen/oxygen concentration dependence of the sulfur distribution among its various available species, as indicated above, results that are *consistent and reproducible can be obtained by using a dual flame system. Problems from variations in concentration changes among the various sulfur containing species unexpectedly do not occur. Interference from hydrocarbon diluents and carrier components give rise to far less signal interference, if any, observed when a dual flame system is used to burn the sulfur containing sample prior to introduction into an ozone reactor, as compared to that interference observed from a single stage flame system.
We have found that when using a single flame burner for the determination of total sulfur content (i.e. no separation of the sulfur components, just a direct injection of the entire sample into the flame) a significant positive interfering signal due to the hydrocarbon matrix was observed. Example 2 illustrates this problem.